Inter vehicle communication systems that communicate with each vehicle are known. In an inter vehicle communication system, a radio communication device mounted on a vehicle exchanges information (current position that is information about running) with radio communication devices respectively mounted on other vehicles (communication partners).
In a radio communication system where radio communication devices directly communicate with each other, not through a base station, such as an inter vehicle communication system, as the radio communication device moves, its communication state changes over time. For example, a communication state, in which there are no communication partners in the communication area its own radio communication device, might change to another communication state in which there are a few communication partners in the communication area its own radio communication device, and then to another communication state in which there are many communication partners in the communication area its own radio communication device.
If there are a large number of communication partners, namely a large number of radio communication devices, in the communication area its own radio communication device, communication of radio communication devices might congest and thereby the throughput might become lower.
Patent Literature 1 describes a radio communication device that can handle this problem. The radio communication device described in Patent Literature 1 controls the transmission power based on the transmission loss of a signal received from the communication partner so as to adjust the size of the communication area. The radio communication device described in Patent Literature 1 adjusts the size of the communication area so as to limit the number of radio communication devices that become communication partners. As a result, while the radio communication device described in Patent Literature 1 maintains a radio link with neighbor radio communication devices, this device can alleviate congestion and the decrease of throughput.